1. Field of the Invention
The present invention relates to operational amplifiers. More particularly, it relates to controlling the transconductance (g.sub.m) of the input stages of an operational amplifier to improve performance and enhance applications.
2. The Prior Art
U.S. Pat. No. 4,555,673 to Huijsing et al. discloses a differential amplifier with rail-to-rail input capability and controlled transconductance (g.sub.m). The method employed for controlling the transconductance uses current control or current switches to steer at least part of the supply current away from at least one of the differential portions of the input stage when the common mode voltage is in at least one part of the supply range.
Other methods for controlling the transconductance (g.sub.m) of the input stages of operational amplifiers with CMOS technology use square root circuits, three times current mirrors, and current switches in order to maintain the g.sub.m of the input stages substantially constant. (Botma, J. H., et al., "A low-voltage CMOS operational amplifier with rail-to-rail constant-gm input stage and class-AB rail-to-rail output stage", Proceeding ISCAS93, pp.1314-1317; Hogervorst, R. et al., "CMOS low-voltage operational amplifiers with constant-gm rail-to-rail input stage", Proceedings ISCAS92, pp.2876-2879; R. Hogervorst, J. P. Tero, R. G. H. Rschauzier, J. H. Huijsing, "A compact power-efficient rail-to-rail input/output amplifier for VLSI cell libraries". in Digest ISSCC94, February 1994.)
The main drawbacks of these existing g.sub.m -controlled rail-to-rail input stages can be found in CMOS technology. Specifically, they are not appropriate for MOS complementary input stages operating in moderate inversion. That is, the transition region between weak and strong inversion. An input stage is frequently biased in moderate inversion because it is a good compromise between a low-offset and a high slew rate.
Another drawback which occurs particularly in CMOS input stages operating in strong inversion, is that the common-mode output currents are 4 times larger at the outer parts of the common-mode input range compared to the intermediate part of the common-mode input range.
Furthermore, the g.sub.m of a CMOS complementary input stage can only be varied over a small range which limits the applicability for which the circuit may be used. In opamps intended for VLSI cell libraries it is often required that the g.sub.m of the input stage be adapted over a large range, so that the specification of the opamp can easily be changed to the need of the specific application.